numpy.triu_indices
返回函数的上三角矩阵
numpy.
triu_indices
(
n,
k=0,
m=None
)
[source]
Return the indices for the upper-triangle of an (n, m) array.
Parameters: n : int
The size of the arrays for which the returned indices will be valid.
k : int, optional
Diagonal offset (see triu for details).
m : int, optional
New in version 1.9.0.
The column dimension of the arrays for which the returned arrays will be valid. By default m is taken equal to n.
Returns: inds : tuple, shape(2) of ndarrays, shape(n)
The indices for the triangle. The returned tuple contains two arrays, each with the indices along one dimension of the array. Can be used to slice a ndarray of shape(n, n).
See also
tril_indices
similar function, for lower-triangular.
mask_indices
generic function accepting an arbitrary mask function.
triu, tril
Notes
New in version 1.4.0.
Examples
Compute two different sets of indices to access 4x4 arrays, one for the upper triangular part starting at the main diagonal, and one starting two diagonals further right:
>>>
>>> iu1 = np.triu_indices(4)
>>> iu2 = np.triu_indices(4, 2)
Here is how they can be used with a sample array:
>>>
>>> a = np.arange(16).reshape(4, 4)
>>> a
array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11],
[12, 13, 14, 15]])
Both for indexing:
>>>
>>> a[iu1]
array([ 0, 1, 2, 3, 5, 6, 7, 10, 11, 15])
And for assigning values:
>>>
>>> a[iu1] = -1
>>> a
array([[-1, -1, -1, -1],
[ 4, -1, -1, -1],
[ 8, 9, -1, -1],
[12, 13, 14, -1]])
These cover only a small part of the whole array (two diagonals right of the main one):
>>>
>>> a[iu2] = -10
>>> a
array([[ -1, -1, -10, -10],
[ 4, -1, -1, -10],
[ 8, 9, -1, -1],
[ 12, 13, 14, -1]])
)
